1. Field of the Invention
The present invention relates to an electric motor and, more particularly, to prevention of irreversible demagnetization which might otherwise result from overheating of the permanent magnets of the rotor.
2. Related Art
One system for cooling a DC brushless motor having permanent magnets arranged around the outer circumference of its rotor directs lubrication oil circulated through the motor onto the coil ends of a stator from an oil passage chamber in the upper portion of a motor case, through oil holes, as disclosed in Japanese Patent Laid-Open No. 122903/1993. In this cooling system, mainly the coil of the stator is cooled to prevent an increase in the electric resistance of the coil, which would change the motor characteristics, as might otherwise result from overheating with feed of a high current.
Permanent magnets arranged around the outer circumference of the rotor are susceptible to becoming irreversibly demagnetized should their temperature rise. In this respect, the oil feed stream from the upper portion of the motor case in the aforementioned prior art can be expected to splash onto the non-rotational parts including the stator. However, in this motor it is difficult to feed the oil to the rotor because the centrifugal force resulting from the rotation of the rotor tends to throw off the oil.